Normally, in the concentrating of tomato juice, multi-effect concentration plants are used which, very briefly, achieve concentration by evaporating the water content of the juice. This is done by regulating the temperature and pressure (depression) by various amounts in the single plant effects. The plants function well but in terms of construction are rather complex and require precise regulating as the temperature and pressure levels in the various effects are of critical importance. Furthermore, for good plant performance, the temperature of some of the effects has to be rather high, with the chance of negative consequences regarding the product itself.
The prior art contains falling film concentration plants which comprise an evaporator provided with a vertical tube bundle in which a plurality of tubes is arranged, ends of which are keyed on two parallel plates, respectively an upper plate and a lower plate, so that the upper ends of the tubes open into an inlet zone of the product, while the lower ends of the tubes open into a bottom zone of the evaporator, known as the separation chamber, in which the juice loses water by self-evaporation (flash), cools, and exits to be sent on to successive operations.
The tubes are closed in a sleeve, generally cylindrical and delimited by the two parallel plates, internally of which a heating fluid circulates, which is generally steam produced by a boiler and subsequently laminated by special valves which reduce pressure and temperature thereof in order to bring the values up to the desired ones. In this way the external surface of the tubes is struck by the heating fluid, while their internal surfaces are occupied by a falling film of product, which, as it heats up, loses water in the form of steam and therefore becomes concentrated. In these plants the tube bundle is often divided into two or more sectors which are all closed within the sleeve of the evaporator and are thus all working at the same temperature. In this case the product which falls from the first section of tubes and arrives at the separation chamber is returned to the top of the tube bundle through a return tube arranged internally of the tube bundle; during its upward return the product, which had cooled in the separation chamber due to evaporation, is heated once more, although it is normally at a slightly lower temperature than the saturation temperature internally of the chamber; and once it has reached to top of the evaporator the product is made to fall in the following sectors. The product is then extracted from the separation chamber, once it has passed through the final section. These plants, and their operation, are established in the prior art.
These plants, which, with respect to multi-effect concentration plants, are easy to regulate and function at lower temperatures, are generally used for low-viscosity products, such as for example clear juices or milk serum, but are not suitable for high-viscosity products and products with a large presence of sugars and fibres, such as for example tomato juice. Though these plants are undoubtedly simple, they do, however present certain drawbacks connected to the impossibility of guaranteeing a homogeneous distribution of the product along the walls of the tubes as well as to the great difficulty of regulating the thickness of the falling film of product as it descends along the internal walls of the tubes. The thickness of the film is, in fact, not regulated in any way which can be called certain and satisfactory, although many attempts have been made to regulate the product inlet delivery or its distribution on the upper plate, from which the product is distributed into the various tubes. Furthermore, the production of steam which occurs inside the tubes is not sufficient to guarantee a regular descent of the product along the internal walls of the tubes, especially the tubes in the sectors subsequent to the first, where the product, due to the concentration achieved in the preceding passages, has a greater density and a slightly lower temperature than that of the operating evaporator, as the product during the return upwards does not reach the internal temperature of the evaporator.